In the development of an electroluminescent device utilizing organic materials, a device developed by optimizing the kind of electrodes and disposing a hole-transporting layer comprising an aromatic diamine and a light-emitting layer comprising 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) between the electrodes for the purpose of improving the efficiency of injecting electrical charges from the electrodes has brought about a noticeable improvement in luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like. Following this, the developmental works of organic EL devices have been focused on their commercial applications to high-performance flat panels characterized by self luminescence and high-speed response.
In order to improve still further the efficiency of an organic EL device such as this, the aforementioned basic structure of anode/hole-transporting layer/light-emitting layer/cathode has been modified by suitably adding a hole-injecting layer, an electron-injecting layer, and an electron-transporting layer. For example, the following structures are known; anode/hole-injecting layer/hole-transporting layer/light-emitting layer/cathode, anode/hole-injecting layer/light-emitting layer/electron-transporting layer/cathode, anode/hole-injecting layer/light-emitting layer/electron-transporting layer/electron-injecting layer/cathode, and anode/hole-injecting layer/hole-transporting layer/light-emitting layer/hole-blocking layer/electron-transporting layer/cathode. The hole-transporting layer has a function of transporting the holes injected from the hole-injecting layer to the light-emitting layer while the electron-transporting layer has a function of transporting the electrons injected from the cathode to the light emitting layer. The hole-injecting layer is sometimes called an anode buffer layer and the electron-injecting layer a cathode buffer layer.
The hole-transporting layer interposed between the light-emitting layer and the hole-injecting layer injects more holes into the light-emitting layer by application of lower electrical field; moreover, the electrons injected into the light-emitting layer from the cathode or from the electron-transporting layer accumulate in the light-emitting layer as the hole-transporting layer obstructs the flow of electrons and, as a result, the luminous efficiency improves.
Likewise, the electron-transporting layer interposed between the light-emitting layer and the electron-injecting layer injects more electrons into the light-emitting layer by application of lower electrical field; moreover, the holes injected into the light-emitting layer from the anode or from the hole-transporting layer accumulate in the light-emitting layer as the electron-transporting layer obstructs the flow of holes and, as a result, the luminous efficiency improves. A large number of organic materials have been developed to perform the function of the constituent layers.
The aforementioned device comprising a hole-transporting layer of an aromatic diamine and a light-emitting layer of Alq3 and many others have utilized fluorescence. Now, the utilization of phosphorescence, that is, emission of light from the triplet excited state, is expected to enhance the luminous efficiency approximately three times that of the conventional devices utilizing fluorescence (singlet). To achieve this object, studies were conducted on the use of coumarin derivatives and benzophenone derivatives for the light-emitting layer, but the result was nothing but extremely low luminance. Thereafter, the use of europium complexes was attempted in trial utilization of the triplet excited state, but it failed to produce high luminous efficiency.
According to a recent report, a device using a platinum complex (PtOEP) can undergo red phosphorescence at high efficiency. Thereafter, a device capable of undergoing green phosphorescence at high efficiency has been fabricated by doping the light-emitting layer with an iridium complex [Ir(ppy)3].
This iridium complex has been found to emit light in a wide wavelength range from blue to red by varying the chemical structure of the ligand. However, it has become evident that a tris complex which is regarded as the most stable complex can be prepared only from a limited number of ligands. To overcome this difficulty, a hetero complex prepared from iridium chloride via an intermediate of cross-linked structure has been proposed (Proceeding of SPIE, vol. 4105, p. 119).
The prior-art documents relating to this invention are listed below.    Patent document 1: JP2002-299061 A    Patent document 2: JP2001-313178 A    Patent document 3: JP2002-352957 A    Patent document 4: JP2003-515897 A    Non-patent document 1: Appl. Phys. Lett., vol. 77, p. 904
In the development of organic EL devices utilizing phosphorescence, 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP) is used as a host material as cited in the patent document 2. However, the use of CBP as a host material for tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3) that is a phosphorescent material emitting green light destroys the balanced injection of electrical charges as CBP has a property of facilitating the flow of holes and obstructing the flow of electrons and excess holes flow out to the side of the electron-transporting layer to lower the luminous efficiency from Ir(ppy)3.
As a means to solve the aforementioned problem, a hole-blocking layer may be disposed between the light-emitting layer and the electron-transporting layer. The hole-blocking layer accumulates holes in the light-emitting layer efficiently, improves the probability of recombination of holes and electrons in the light-emitting layer, and enhances the luminous efficiency. The hole-blocking materials currently in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato-N1,O8)aluminum (hereinafter referred to as BAlq). A hole-blocking material such as this can prevent electrons and holes from recombining in the electron-transporting layer. However, BCP tends to crystallize easily at room temperature and a device comprising BCP lacks reliability and shows an extremely short life. On the other hand, BAlq is reported to show a relatively long life, but lacks a sufficient hole-blocking ability and lowers the luminous efficiency from Ir(ppy)3. In addition, an increase of one layer complicates the layered structure of the device and increases the cost.
The use of 3,5-diphenyl-4-(1-naphthyl)-1,2,4-triazole (hereinafter referred to as TAZ) as a host material for a phosphorescent organic EL device is proposed in the patent document 3; however, the light-emitting range is displaced toward the side of the hole-transporting layer as TAZ has a property of facilitating the flow of electrons and obstructing the flow of holes. Hence, the luminous efficiency from Ir(ppy)3 may drop depending upon the compatibility of the material used for the hole-transporting layer with Ir(ppy)3. For example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as α-NPD) that is a material most widely used as a hole-transporting layer for its excellent performance, high reliability, and long life shows poor compatibility with Ir(ppy)3 and problems arise in that energy transition occurs from TAZ to α-NPD, the efficiency of energy transition from Ir(ppy)3 to Ir(ppy)3 drops, and the luminous efficiency drops.
As a means to solve the aforementioned problem, a material to which energy transition from Ir(ppy)3 does not occur, for example, 4,4′-bis[N,N′-(3-toluoyl)amino]-3,3′-dimethylbiphenyl (hereinafter referred to as HMTPD), is used for the hole-transporting material. It is reported in the non-patent document 1 that a phosphorescent EL device of a three-layer structure consisting of a light-emitting layer comprising TAZ, 1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxazole, or BCP as a host material and Ir(ppy)3 as a guest material, an electron-transporting layer comprising Alq3, and a hole-transporting layer comprising HMTPD can emit light at high efficiency and a system using TAZ performs particularly well. However, HMTPD tends to crystallize easily as it has a glass transition temperature (Tg) of approximately 50° C. and lacks reliability as a material for the hole-transporting layer. Thus, a device comprising HMTPD faces problems such as extremely short life, difficulty of commercial application, and high driving voltage.
The patent document 1 discloses an organic EL device comprising a light-emitting layer containing a host material and a phosphorescent doping material and cites Ir (ppy)3 and a tris complex in which Ir is coordinated to a ligand having a phenyl-substituted benzothiazole structure as examples of such doping materials.